1. Field of the Invention
The present invention relates to a power converter, and more specifically, relates to a control circuit of switching power converter.
2. Description of Related Art
Power supplies have been widely used to provide regulated output voltages. For safety, an off-line power supply must provide isolation between its primary side and secondary side. An optical-coupler and a secondary-side regulator are needed to regulate the output voltage of the off-line power supply. In order to reduce the device counts and the secondary-side feedback circuit, the reflected voltage of the transformer has been utilized to regulate the output voltage of the power supply, for example, “Rectifier-converter power supply with multi-channel flyback inverter”, by Randolph D. W. Shelly, U.S. Pat. No. 4,302,803. However, this skill cannot measure an accuracy voltage signal from the transformer, which causes a poor regulation of the power converter. Therefore, many primary-side control techniques are developed recently for providing a precise voltage measurement to the transformer, such as “Primary-side controlled flyback power converter” by Yang, et al, U.S. Pat. No. 6,853,563; “Close-loop PWM controller for primary-side controlled power converters” by Yang, et al, U.S. Pat. No. 7,016,204. However, the detection circuits of these prior arts are complicated, which increase the cost of power supply. How to overcome these shortcomings are the main object of the present invention.